The present invention relates to a fluid passage change over valve unit for use in concrete pumps which pump freshly-mixed or ready-mixed concrete to a casting or deposite site such as building or construction site.
FIG. 1 illustrates a typical well-knonw concrete pump. The concrete pump comprises a hopper 1 into which a ready-mixed concrete A is fed, a pump unit having a pair of piston cylinders 3 and 4 which are mounted to a rear wall 2 of hopper 1 to communicate to the inside of hopper 1 and perform the intake and discharge of fluid concrete A, and a valve unit for changing over a fluid passage from the pump cylinders 3 and 4 to a delivery line (not shown) which leads to a deposite site, the valve unit having a pivotal valve pipe 5, one end of which is connected to the delivery line for pivoting about the latter and the other end of which is pivoted for alternate communication with the pump cylinders 3 and 4.
In the prior concrete pump, for example, the retraction of a piston of one cylinder 4 causes the concrete A to be drawn into that cylinder. The swinging pipe 5 is then moved into engagement with the charged cylinder 4 and the piston is extended to force the concrete through the swing pipe 5 and into the delivery line. During such operation, the other cylinder 3 is filling with the fluid concrete A for subsequent discharge into valve pipe 5 during the charging of the first cylinder 4. In such manner, the swing pipe 5 alternately connects to the cylinders 3 and 4, whereby the fluid concrete A is pumped from hopper 1 through swing pipe 5 and delivery line to a deposit site.
Such conventional concrete pump has the following drawbacks. Usually, swing pipe 5 is instantaneously switched and hence it is necessary to move, in quick response to the switching of pipe 5 a large amount of fluid concrete A within hopper 1 toward the opening of cylinder 3 or 4 which is to be filled with the fluid concrete for facilitating the drawing of the concrete into the cylinder. However, when the ready-mixed concrete within the hopper 1 is of a low slump, it cannot follow the quick swinging motion of pipe 5 since the concrete is fed to the cylinders 3 and 4 by gravity, and thus a vacuum portion is momentarily formed around the opening of cylinder 3 or 4 which is out of communication with swinging pipe 5 as shown by a dot and dash line in FIG. 2. This can separate the fluid concrete into cement paste and aggregate. Furthermore arches of the fluid concrete can be formed above the openings of cylinders 3 and 4 within hopper 1 as at 8 in FIG. 2, with the result that the concrete at the arch 8 is prevented from dropping down and from being fed to the opening. In addition, air passages from the openings of cylinders 3 and 4 to the atmosphere can be formed in the fluid concrete A within hopper 1, which causes air to be drawn in the cylinders 3 and 4 during their intake stroke. Such drawbacks will reduce the intake of concrete A into cylinders 3 and 4, and hence lower the transfer efficiency of the concrete pump. Not only for low slump concrete but also for fluid concrete which is liable to be separated into cement paste and aggregates, the ready-mixed concrete A which is fed to the cylinders 3 and 4 can be substantially restricted to two funnel-shaped portions in the concrete A which extend upwardly from the openings of pump cylinders 3 and 4 respectively, with the result that funnel-shaped air passages are formed, communicating the openings to the atmosphere. Thus, the remainder of the fluid concrete A can not be drawn in cylinders 3 and 4 and further the cement paste and aggregates of the concrete A can be separately drawn in the cylinders. These disadvantages can cause reduction in intake efficiency of the fluid concrete into the pump cylinders and the choking of the swing pipe 5 or the delivery line due to the pumping of only aggregate component.
To overcome the above disadvantages there has been proposed a concrete pump having an agitator 6 with paddles 7 as shown in FIG. 2, the agitator 6 being usually disposed within hopper 1 above the openings of cylinders 3 and 4. The rotation speed of paddles 7 is restricted to a speed much smaller than that of swinging pipe 5 because increase in speed of paddles 7 beyond the limited speed will produce a considerable loss of power, separation of the concrete into cement paste, aggregates, etc., and thus it is impracticable to feed a large amount of the ready-mixed concrete A by paddles 7 to the cylinder openings in quick response to swing motion of valve pipe 5, with the result that agitator 6 cannot sufficiently eliminate the disadvantages at least in a low slump concrete. In addition, the agitator paddles 7 are likely to block instantaneous movement of fluid concrete A of a low slump whereby the disadvantages can be enlarged.
Further, when valve pipe 5 is pivoted, it forces adjacent fluid concrete toward and substantially perpendicularly to the side walls of hopper 1, so that valve pipe 5 can hold the fluid concrete against the side walls if the fluid concrete A is largely separated and contains a relatively large proportion of aggregates, and thus the switching of valve pipe 5 can be adversely affected or cannot be performed at its worst.